[web] New platform view API to get view by ID

lib/web_ui/lib/src/engine/dom.dart

@@ -520,6 +520,8 @@ extension DomElementExtension on DomElement {
   Iterable<DomElement> get children =>
       createDomListWrapper<DomElement>(_children);
 
+  external DomElement? get firstElementChild;
+
   @JS('clientHeight')
   external JSNumber get _clientHeight;
   double get clientHeight => _clientHeight.toDart;

lib/web_ui/lib/src/engine/platform_views/content_manager.dart

@@ -60,6 +60,17 @@ class PlatformViewManager {
     return _contents.containsKey(viewId);
   }
 
+  /// Returns the HTML element created by a registered factory for [viewId].
+  ///
+  /// Throws an [AssertionError] if [viewId] hasn't been rendered before.
+  DomElement getViewById(int viewId) {
+    assert(knowsViewId(viewId), 'No view has been rendered for viewId: $viewId');
+    // `_contents[viewId]` is the <flt-platform-view> element created by us. The
+    // first (and only) child of that is the element created by the user-supplied
+    // factory function.
+    return _contents[viewId]!.firstElementChild!;
+  }
+
   /// Registers a `factoryFunction` that knows how to render a Platform View of `viewType`.
   ///
   /// `viewType` is selected by the programmer, but it can't be overridden once
@@ -119,12 +130,13 @@ class PlatformViewManager {
             ..setAttribute('slot', slotName);
 
       final Function factoryFunction = _factories[viewType]!;
-      late DomElement content;
+      final DomElement content;
 
       if (factoryFunction is ParameterizedPlatformViewFactory) {
         content = factoryFunction(viewId, params: params);
       } else {
-        content = (factoryFunction as PlatformViewFactory).call(viewId);
+        factoryFunction as PlatformViewFactory;
+        content = factoryFunction(viewId);
       }
 
       _ensureContentCorrectlySized(content, viewType);

lib/web_ui/lib/ui_web/src/ui_web/platform_view_registry.dart

@@ -23,4 +23,11 @@ class PlatformViewRegistry {
       isVisible: isVisible,
     );
   }
+
+  /// Returns the view previously created for [viewId].
+  ///
+  /// Throws if no view has been created for [viewId].
+  Object getViewById(int viewId) {
+    return platformViewManager.getViewById(viewId);
+  }
 }

lib/web_ui/test/engine/platform_views/content_manager_test.dart

@@ -145,5 +145,63 @@ void testMain() {
         expect(firstRender, same(anotherRender));
       });
     });
+
+    group('getViewById', () {
+      test('finds created views', () async {
+        final Map<int, DomElement> views1 = <int, DomElement>{
+          1: createDomHTMLDivElement(),
+          2: createDomHTMLDivElement(),
+          5: createDomHTMLDivElement(),
+        };
+        final Map<int, DomElement> views2 = <int, DomElement>{
+          3: createDomHTMLDivElement(),
+          4: createDomHTMLDivElement(),
+        };
+
+        contentManager.registerFactory('forTest1', (int id) => views1[id]!);
+        contentManager.registerFactory('forTest2', (int id) => views2[id]!);
+
+        // Render all 5 views.
+        for (final int id in views1.keys) {
+          contentManager.renderContent('forTest1', id, null);
+        }
+        for (final int id in views2.keys) {
+          contentManager.renderContent('forTest2', id, null);
+        }
+
+        // Check all 5 views.
+        for (final int id in views1.keys) {
+          expect(contentManager.getViewById(id), views1[id]);
+        }
+        for (final int id in views2.keys) {
+          expect(contentManager.getViewById(id), views2[id]);
+        }
+
+        // Throws for unknown viewId.
+        expect(() {
+          contentManager.getViewById(99);
+        }, throwsA(isA<AssertionError>()));
+      });
+
+      test('throws if view has been cleared', () {
+        final DomHTMLDivElement view = createDomHTMLDivElement();
+        contentManager.registerFactory(viewType, (int id) => view);
+
+        // Throws before viewId is rendered.
+        expect(() {
+          contentManager.getViewById(viewId);
+        }, throwsA(isA<AssertionError>()));
+
+        contentManager.renderContent(viewType, viewId, null);
+        // Succeeds after viewId is rendered.
+        expect(contentManager.getViewById(viewId), view);
+
+        contentManager.clearPlatformView(viewId);
+        // Throws after viewId is cleared.
+        expect(() {
+          contentManager.getViewById(viewId);
+        }, throwsA(isA<AssertionError>()));
+      });
+    });
   });
 }